Detection of plunger movement in dc solenoids through current sense technique

ABSTRACT

An apparatus and method of detecting movement of a plunger of the solenoid includes detecting a peak (I PEAK ) in a current signal applied to a coil of the solenoid. A predetermined threshold is added to the current signal applied to the coil of the solenoid to generate a level shifted signal. The level shifted signal and the peak signal are compared to detect movement of a plunger of the solenoid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Provisional Indian PatentApplication No. 5512/CHE/2014, filed Nov. 3, 2014, which is incorporatedherein by reference in its entirety for all purposes.

FIELD

The invention generally relates to a solenoid controller and, morespecifically to a solenoid detector that can detect complete plungermovement

BACKGROUND

Linear solenoids are electromechanical devices that convert electricalenergy into a linear mechanical motion, which is used to controlelectrical, pneumatic or hydraulic systems. Solenoids are used invalves, relays and contactors.

Electromechanical solenoids consist of an electromagnetically inductivecoil that is wound to encircle a movable steel or iron slug, termed “thearmature” or “plunger.” The coil is shaped such that the plunger can bemoved in and out of its center, altering the inductance of the coil. Theplunger is used to provide a mechanical force to activate the controlmechanism, for example opening and closing of a valve.

A solenoid coil needs a higher current during activation, called thepull—in current, to pull the plunger into the solenoid. However, oncethe plunger has moved completely, the solenoid coil needs onlyapproximately 30% of its nominal current, called “the hold current,” tokeep the plunger in the same position. DC solenoids having coils thatoperate continuously at their nominal current, which is limited by theresistance of the coil, will have an increase in temperature of the coildue to the higher power dissipation. Once the complete plunger movementis detected, the steady-state current can be reduced to the hold currentto minimize the power consumption in the solenoid. The detection of theplunger movement is required in safety-critical applications to detectproper operation of the valve, relays or contactors. Movement of theplunger can be slow, due to factors such as friction, rusting and othermechanical impediments to the movement of the plunger.

FIG. 1 shows an example of a known solenoid drive circuit, showngenerally as 100. A DC input voltage at 102 is applied to one terminalof a solenoid coil 104, the other terminal of the solenoid coil isconnected to a transistor 108, controlling current through the solenoid,which is sensed by sense resistor R_(SENSE) 110. Transistor 108 iscontrolled by current controlled solenoid driver 112, which will drivethe solenoid at its nominal current until the plunger has movedcompletely, at which time the current can be reduced to its hold value.Freewheeling diode 106 is used to eliminate the sudden voltage spikeseen across the transistor when it is switched off by the currentcontrolled solenoid driver.

FIG. 2 shows the known excitation current waveform of a solenoid,generally as 200. As soon as the solenoid is energized at 202, thecurrent begins to increase as shown at 204. When the current reachesI_(PEAK) at 206, the plunger starts moving because of the sufficientmagnetic field created by the solenoid coil. The movement of the plungerinduces back EMF in the coil, and hence, the solenoid current startsdropping. At 208, the plunger has moved completely and the current dipsto I_(VALLEY). After the plunger strokes, the current continues on itsnormal upward path, as shown at 210, to its maximum value, as shown at212, which is limited by the resistance of the coil. The prominent dipin the excitation curve from I_(PEAK) to I_(VALLEY) is an indication ofplunger movement.

One known plunger position sensing method includes hall sensors todetect the position of the plunger. The mechanical mounting of thesesensors are complex and their performance is affected by ageing andexternal field. In addition, the hall sensor will provide a signal atthe end of the plunger movement, and therefore, cannot detect slowmovement of the plunger.

Other plunger movement detection logic uses fixed references fordetecting peak and valley current, or utilize algorithmic solutions.These algorithms may fail during temperature variation or during slowmovement of plunger.

There is a need for a simple, low-cost and reliable technique fordetecting complete solenoid movement that can detect plunger movementover wide variation of temperature and also detect slow-moving plungers.

SUMMARY

A method of detecting movement of a plunger of the solenoid, includingdetecting a peak (I_(PEAK)) in a current signal applied to a coil of thesolenoid. A valley (I_(VALLEY)) in a current signal applied to a coil ofthe solenoid is detected. A predefined threshold is defined as theabsolute difference between the peak and valley current signal. Thepredetermined threshold is added to the current signal applied to thecoil of the solenoid to generate a level shifted signal. The levelshifted signal and the peak signal are compared to detect movement of aplunger of the solenoid.

In a system for operating a solenoid, a circuit to detect completemovement of the plunger of the solenoid, includes a device for measuringcurrent through the solenoid and generating a first current signal. Anactive peak detector circuit receives the first current signal fordetecting a peak thereof and generating a peak detection signal. A levelshifter circuit is coupled in parallel with the active peak detector andreceives the first current signal, the level shifter circuit adding athreshold voltage to the first current signal to generate a levelshifted signal. A comparator compares the peak detection signal with thelevel shifted signal, whereby if the level shifted signal matches thepeak detection signal, complete movement of the solenoid is detected.

A control circuit for operating a solenoid includes a circuit applying avoltage across a solenoid coil and measuring current through the coil togenerate a first signal. A detector circuit detects a peak in thecurrent through the coil represented by the first signal. An addercircuit adds a predetermined threshold to the first signal. A comparatorcompares the output of the adder circuit to the output of the detectorcircuit to detect movement of the plunger of the solenoid.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the invention will appear from the appending claimsand from the following detailed description given with reference to theappending drawings:

FIG. 1 shows an example of a known solenoid drive circuit;

FIG. 2 shows the excitation current of a solenoid;

FIG. 3 shows the current drawn by the solenoid at differenttemperatures;

FIG. 4 shows the difference between I_(PEAK) and I_(VALLEY) (predefinedthreshold);

FIG. 5 is a block diagram of solenoid plunger position detection logic;

FIG. 6 shows the plunger position detection circuit consisting of anamplifier, active peak detector, op-amp adder circuit and a comparatorwith latch;

FIG. 7 shows the peak detector circuit;

FIG. 8 shows the solenoid current level shifting circuit;

FIG. 9 shows the comparator circuit generating the signal I_(TRIP);

FIG. 10 shows plunger movement circuit waveforms;

FIG. 11 shows finding the threshold from the solenoid excitation currentcharacteristics; and

FIG. 12 compares the excitation characteristics of a normal and aslow-moving plunger in a solenoid.

DETAILED DESCRIPTION

The method and apparatus disclosed herein is based on change in currentwave shape due to back EMF generated by plunger movement. In anembodiment, the detection circuit comprises of current sense amplifier,peak detector, level shifter and comparator.

The current, drawn by the solenoid just before start of plungermovement, is held by a peak detector. The circuit tracks the dip incurrent due to the back EMF generated by the plunger movement. If thedip in current is more than the predefined threshold, it is interpretedas complete movement of plunger. The threshold is set based on plungercharacteristics.

If the solenoid is faulty or if the plunger did not move fully, or movedvery slowly, then the circuit can generate a fault signal I_(TRIP) thatis equal to zero.

In order to study the effect of temperature on the excitation current, atypical solenoid is characterized at different temperatures. FIG. 3shows the current drawn by the solenoid at 3 different temperatures,−30° C., 0° C. and +45° C. The curves shift up as the temperaturedecreases because of the reduction in resistance of the solenoid coil.However, it should be noticed that the difference between the peak(I_(PEAK)) and valley (I_(VALLEY)) of the solenoid current, due to backEMF, remains constant irrespective of the temperature. Thischaracteristic of the solenoid current is utilized by the presentdisclosure in detecting the movement of the plunger of the solenoid.

Referring now to FIG. 4, a solenoid current characteristic is showngenerally as 400. When a voltage is applied to the solenoid, the currentrises at 404 to a peak 406 and then drops back to a valley at 408 due tothe back EMF of the solenoid coil. Once the plunger moves completely,the current will increase, as shown at 410, until it reaches its maximumat 412, which is determined by the coil resistance. The absolutedifference between I_(PEAK) at 406 and I_(VALLEY) at 408 is measured andis used to define the threshold for the detection of plunger movement.The threshold is set slightly above I_(VALLEY) and is referenced toI_(PEAK) so as to reduce the effect of temperature on the detectioncircuit.

FIG. 5 shows a block diagram of a plunger movement detection circuitconsisting of a gain stage, an active peak detector, an op-amp addercircuit and a comparator. This detection logic can be implementedutilizing simple op-amp circuits rather than using a sensor orcontroller. The solenoid current is detected using the sense resistorR_(SENSE) shown in FIG. 1, which is applied to gain stage 504 at 502.The voltage across the resistor is filtered and amplified by the gainstage 504, which is configured as a differential amplifier, amplifyingthe voltage across the sense resistor to the suitable high-value inorder to improve the noise immunity of the following circuits. Theoutput of the differential amplifier gain stage 504, is shown bywaveform 520, and is fed simultaneously to the active peak detector 514and buffer 508. The active peak detector 514 is used to track thedifferential amplifier 504 output corresponding to the peak current(I_(PEAK)) during the excitation of the solenoid. The output of the peakdetector is shown at 526.

Buffer circuit 508 and adder circuit 510 form a level shifter circuit512, which is used to introduce a positive level shift equivalent to thepredefined threshold 414 in FIG. 4. The threshold value that is added byadder circuit 510 to the output of buffer 508 is shown at 506. Theoutput of the level shifter circuit is shown at 522. The signals shownat 522 and 526 are input to the comparator 516. The output of the addercircuit is input to the inverting input of the comparator 516 and theoutput of the active peak detector 514 is input to the non-invertinginput of the comparator 516. Adding the threshold to the output of thegain stage 504 and comparing this to the output of the active peakdetector results in a trip signal on the output 518 of comparator 516,when the two voltages substantially match. The output of the active peakdetector 514 will store the value of the first peak such as peak 406shown FIG. 4. As the solenoid current 400 drops to form the valley at408, the value of the signal 520, plus the threshold, will substantiallymatch the peak value of the signal 526, at the instant 408, triggeringthe comparator to produce the trip signal I_(TRIP) 518. The trip signal518 indicates that the plunger has completely moved within the solenoid.

FIG. 6 shows a schematic diagram of block diagram 500, generally as 600.In FIG. 6, amplifier 604 receives the voltage from the current senseresistor R_(SENSE) shown in FIG. 1 and as resistor R34 in FIG. 6, at theI_(SENSE)+ and −I_(SENSE)− terminals. Differential amplifier 604amplifies this voltage in order to increase noise immunity in thefollowing circuits. Resistor R8, capacitor C1, and resistor R1, form alow pass filter to smooth the voltage developed across R_(SENSE). Theoutput of amplifier 604 is fed simultaneously to active peak detector614 and buffer circuit 608. The output of the buffer circuit 608 is fedto adder circuit 610 which adds the threshold value to the output of theamplifier 604. The output of the adder circuit is fed to the invertingterminal of comparator 616. The output of the peak detector 614 is fedto the non-inverting input of comparator 616. Comparator 616 produces asignal I_(m)p which identifies that the plunger of the solenoid hasmoved completely.

The peak detector circuit shown in FIG. 6 is shown separately in FIG. 7,generally as 700. The output of differential amplifier U2A is fed to theactive peak detector formed by the op-amp U2B, diode D2, resistor R26and capacitor C4. The peak detector detects the maximum value of thesignal over a period of time. A simple diode and capacitor can form apeak detector; however, adding an op-amp with feedback can eliminate thediode drop and make a more precise peak detector.

The output of differential amplifier U2A at ISENSE-AMP is fed to thenon-inverting input of the op-amp U2B. The output of the op-amp U2B isconnected to capacitor C4 through diode D2 and resistor R26. The nodeformed by resistor R26 and capacitor C4 is connected to the invertinginput of op-amp U2B. The high differential gain of op-amp U2B causes thecapacitor C4 to charge to a voltage that equals the non-inverting inputvoltage of the op-amp.

The presence of diode D2 and the ultra-low input bias current of op-ampU2B assures that the capacitor C4 will not discharge even if thenon-inverting input of the op-amp U2B goes below the voltage acrosscapacitor C4. This means that the voltage across capacitor C4 willalways track the maximum value at the non-inverting terminal of theop-amp U2B.

A small resistor R26 is provided to increase the stability of op-amp U2Bas it is charging the capacitor. VIN is the power supply to the op-amp.During times when the power is off, the VIN voltage suddenly reduces tozero. The diode D1 makes sure that during a power off situation, thecapacitor C4 will discharge immediately, which helps tracking thesolenoid current during the next power on sequence.

The level shifter circuit 612 of FIG. 6, is shown separately in FIG. 8,generally as 800. The op-amp U2C is the voltage buffer 608 and theop-amp U2D is the adder circuit 610. The two op-amps, U2C and U2D, areused to generate the level shifted waveform of the current sense signal.The adder circuit is used to provide a level shifting equal to thethreshold defined by the characterization current curve for thesolenoid. The threshold is the voltage equivalent of the differencebetween the peak and valley currents defined in FIG. 4. The op-amp U2Cis used as a buffer. The buffer circuit ensures that the differentialamplifier will not be loaded by the circuit components in the addercircuit. The function of the buffer U2C is to replicate the signalprovided at its input to its output. The buffer offers a high inputimpedance and a low output impedance. The input of the buffer is thedifferential amplifier (U2A) output. This signal will then appear at theoutput of the buffer U2C.

The adder circuit is formed by op-amp U2D, resistors R18, R19, R12 andR11. The threshold can be set by adjusting the values of the resistors.

By selecting R11=R19 and R12=R18:

${{Output}\mspace{14mu} {of}\mspace{11mu} U\; 2D} = {I_{SENSE\_ AMP} + ( {V_{IN}*\frac{R\; 11}{R\; 12}} )}$

This equation allows the values of the resistors to be designed to addthe required threshold to the output of the buffer. This means that thethreshold is added to the output of the buffer to generate the levelshifted waveform.

The comparator 616 shown in FIG. 6 is shown separately in FIG. 9,generally as 900. The resistor R29 is used to avoid false latching ofthe comparator U3 during power-on. The components resistor R17 andcapacitor C11 (see FIG. 6) also avoid power-on latching of thecomparator U3. The resistor R17 will pull the inverting input of U3 to apositive voltage during power on, which depends upon the value of R17and R14 (see FIG. 6), to make sure that the inverting input of U3 isgreater than its non-inverting input to make sure that there will be nolatching of U3 during power-on of the comparator.

In operation, the peak detector output is connected to the non-invertinginput of the comparator through the diode D3 and resistor R28. The levelshifted signal is connected to the inverting input of the comparator.VIN is a supply voltage for the comparator. The output of the comparatorU3 will be a logic high, equal to VIN, when the non-inverting inputvoltage of U3 is higher than the inverting input voltage. Similarly, theoutput of the comparator U3 would be zero when the non-inverting inputvoltage of U3 is less than the inverting input voltage. The diode D8will be reverse biased when the output of U3 is zero. The output of thecomparator U3 is the signal

At the start of the solenoid energization, the peak detector outputvoltage will be less than the level shifted signal. Therefore, theoutput of comparator U3 will be zero. When the solenoid plunger movescompletely, the output of the peak detector would be higher than thelevel shifted signal, which makes the non-inverting input voltage of U3higher than the inverting input voltage. Therefore, the output of U3goes high. That means, the signal I_(TRIP) goes to the voltage levelVIN. This causes diode D8 to be forward-biased and hence non-invertinginput of U3 becomes equal to the output voltage, which is VIN, minus thediode drop. This ensures that once the output of the comparator goeshigh, the non-inverting input is always higher than the inverting inputand therefore the output latches to high. In other words, on completemovement of the solenoid plunger, the signal I_(TRIP) goes from zero tohigh and latches there. The latching of I_(TRIP) at the high-valueensures that any other monitoring circuit provided to monitor I_(TRIP),will have enough time to process the signal. The state of I_(TRIP) atVIN implies that the plunger has moved completely. The state of I_(TRIP)at zero implies that the plunger has not moved or the plunger is faulty.

FIG. 10 shows a detail of the working principle of the circuit. Theoutput of the differential amplifier U2A, which is the amplifiedsolenoid current, is fed to the peak detector. The peak detector outputtracks the solenoid current until point 1, where the solenoid plungerstarts moving. After this point, the solenoid current decreases becauseof the back EMF generated by the solenoid and the solenoid current dipsto point 2. The output of the peak detector will remain at a value equalto the peak value at point 1 and is fed to the non-inverting input ofthe comparator U3. The inverting input of the comparator U3 is fed withthe level shifted solenoid current signal. This waveform is derived bylevel shifting the solenoid current waveform by a voltage equal to thedifference between the peak current at point 1 and the valley current atpoint 2, the threshold. At point 2, the non-inverting input voltage ofthe comparator (the peak detector output) becomes higher than theinverting input voltage (the output of the op-amp adder circuit) and thecomparator output goes high and latches there.

Referring back to FIG. 3 which shows the current drawn by the solenoidat different temperatures, remember that the curves shift up astemperature decreases because of the reduction in the resistance of thesolenoid coil. As stated in the description of FIG. 3, the differencebetween the peak and the valley of the solenoid current dip due to theback EMF remains constant irrespective of the temperature. The absolutedifference between I_(PEAK) and I_(VALLEY) is measured and used todefine the threshold for detection of plunger movement for a particularsolenoid. The non-inverting input of the comparator is the output fromthe peak detector and the inverting input is the level shifted solenoidcurrent waveform. The active peak detector output connected to thecomparator ensures that the threshold is always referred to the peakcurrent which makes the logic more immune to temperature variation. InFIG. 10, at point 2, in order to make sure that the level shiftedwaveform falls below the peak detector output, for the comparatortrigger and latch, the threshold is referenced slightly aboveI_(VALLEY). This ensures that the comparator inverting input will not behigher than the noninverting input voltage for sufficient time, such asmore than 1 μs, for the comparator to act. When the solenoid currentdrops below the predetermined threshold, the comparator output will gohigh indicating that the plunger has moved completely.

FIG. 11 shows how the threshold is determined for a solenoid. Thethreshold is found by using the excitation current characteristics of a“healthy” solenoid. A typical solenoid is tested, experimentally, tofind out the current characteristics. The rated DC voltage is appliedacross the solenoid, the excitation current waveform is captured and thedifference between the peak and valley currents are measured as shown inFIG. 11.

Prolonged operation of solenoid can cause a plunger movement to becomeslow due to factors such as friction, rusting and other factors. Forexample, if the plunger is expected to move 10 mm within 10 ms, and ifit does not move at the same speed as expected, the back EMF, generatedin the solenoid coil, will be less. Hence, the magnitude of the currentdip will not be equal to that of a “healthy” solenoid. As shown in FIG.12, generally as 1200, when the plunger movement is slow, that is thetime T2 is greater than T1, the magnitude of the current dip I_(DIP2)will be less than the current dip I_(DIP1), as shown in FIG. 12. Thiswill cause the waveform to have a delayed and lower dip as shown in FIG.12 at time T2, which causes the comparator output to remain at zero.This can be utilized to detect a faulty solenoid.

Although the invention has been described in detail, it should beunderstood that various changes, substitutions and alterations, may bemade thereto without departing from the spirit or scope of the inventionas defined by the appended claims.

1. A method of detecting movement of a plunger of the solenoidcomprising: detecting a peak (I_(PEAK)) in a current signal applied to acoil of the solenoid; detecting a valley in the current signal appliedto the coil of the solenoid to generate a valley signal (I_(VALLEY));adding a predetermined threshold to the current signal applied to thecoil of the solenoid to generate a level shifted signal; comparing thelevel shifted signal and the peak signal to detect movement of a plungerof the solenoid.
 2. The method of claim 1 wherein the threshold isrelated to an absolute value of the difference between I_(PEAK) andI_(VALLEY).
 3. The method of claim 2 within the threshold is determinedby measuring characteristics of the solenoid.
 4. The method of claim 1wherein if the current signal does not have a dip at least equal to thepredetermined threshold, a fault signal is generated.
 5. The method ofclaim 1 wherein if the current signal has a dip at least equal to thepredetermined threshold, current to the solenoid is reduced to its holdvalue.
 6. The method of claim 5 wherein the threshold is added to avoltage representing coil current which is compared with an output of anactive peak detector, if the level shifted voltage matches the output ofthe active peak detector, complete solenoid movement has been detected.7. The method of claim 4 wherein a dip less than the threshold indicatesan unacceptably slower plunger movement.
 8. In a system for operating asolenoid, a circuit to detect complete movement of the plunger of thesolenoid, comprising: a device for measuring current through thesolenoid and generating a first current signal; an active peak detectorcircuit receiving the first current signal for detecting a peak thereofand generating a peak detection signal; a level shifter circuit coupledin parallel with the active peak detector and receiving the firstcurrent signal, the level shifter circuit adding a threshold voltage tothe first current signal to generate a level shifted signal; and acomparator comparing the peak detection signal with the level shiftedsignal, whereby if the level shifted signal matches the peak detectionsignal, complete movement of the solenoid is detected.
 9. The system ofclaim 8 wherein the threshold is related to an absolute value of thedifference between a current peak and a current valley.
 10. The systemof claim 8 wherein the threshold is determined by measuringcharacteristics of the solenoid.
 11. The system of claim 8 wherein ifthe first current signal does not have a dip at least equal to thepredetermined threshold, a fault signal is generated.
 12. The system ofclaim 8 wherein if the first current signal does not have a dip at leastequal to the predetermined threshold, the solenoid current controllercontinues to drive the solenoid at its nominal current.
 13. The systemof claim 8 wherein if the first current signal has a dip at least equalto the predetermined threshold, current to the solenoid is reduced toits hold value.
 14. The system of claim 8 wherein the threshold voltageis added to a voltage representing coil current which is compared withan output of an active peak detector, if the level shifted voltagematches the output of the active peak detector, complete solenoidmovement has been detected.
 15. A control circuit for operating asolenoid comprising: a circuit for applying a voltage across a solenoidcoil and measuring current through the coil to generate a first signal;a detector circuit detecting a peak in the current through the coilrepresented by the first signal; a circuit for detecting a valley in thefirst signal; an adder circuit for adding a predetermined threshold tothe first signal; a comparator comparing output of the adder circuit tothe peak signal to detect movement of the plunger of the solenoid. 16.The control circuit of claim 15 wherein the threshold is related to anabsolute value of the difference between the peak and valley currents.17. The control circuit of claim 16 wherein the threshold is determinedby measuring characteristics of the solenoid.
 18. The control circuit ofclaim 15 wherein the current signal does not have a dip at least equalto the predetermined threshold, a fault signal is generated.
 19. Thecontrol circuit of claim 15 wherein if the first current signal does nothave a dip at least equal to the predetermined threshold, the solenoidcurrent controller continues to drive the solenoid at its nominalcurrent.
 20. The control circuit of claim 15 wherein if the currentsignal has a dip at least equal to the predetermined threshold, currentto the solenoid is reduced to its hold value.
 21. The control circuit ofclaim 15 wherein the threshold is added to a voltage representing coilcurrent which is compared with a output of active peak detector, if thelevel shifted voltage matches the output of the active peak detector,complete solenoid movement has been detected.
 22. The control circuit ofclaim 18 wherein a dip less than the threshold indicates an unacceptablyslower plunger movement.